Multiline snippet ghost text with dynamic window position flip

[xzbdmw]

follow up of #1886 fix https://github.com/hrsh7th/nvim-cmp/pull/1886#issuecomment-2156131192 close #495

should behave the same as vscode now. Custom snippet not supported yet, only lsp snippet and copilot-similar source will trigger multi-line ghost-text, but should be easy to add.

The functionality is all implemented, @hrsh7th do let me know what do you think, Thanks!

https://github.com/hrsh7th/nvim-cmp/assets/97848247/58bc858d-5674-480d-96b0-51e424987a28